Pattern Transfer

Lithography in the MEMS context is typically the transfer of a
pattern to a photosensitive material by selective exposure to a
radiation source such as light. A photosensitive material is a
material that experiences a change in its physical properties when
exposed to a radiation source. If we selectively expose a
photosensitive material to radiation (e.g. by masking some of the
radiation) the pattern of the radiation on the material is transferred
to the material exposed, as the properties of the exposed and
unexposed regions differs (as shown in figure 1).

Figure 1:Transfer of a pattern to a photosensitive material.

This discussion will focus on optical lithography, which is simply
lithography using a radiation source with wavelength(s) in the visible
spectrum.

In lithography for micromachining, the photosensitive material used
is typically a photoresist (also called resist, other photosensitive
polymers are also used). When resist is exposed to a radiation source
of a specific a wavelength, the chemical resistance of the resist to
developer solution changes. If the resist is placed in a developer
solution after selective exposure to a light source, it will etch away
one of the two regions (exposed or unexposed). If the exposed
material is etched away by the developer and the unexposed region is
resilient, the material is considered to be a positive resist (shown
in figure 2a). If the exposed material is resilient to the developer
and the unexposed region is etched away, it is considered to be a
negative resist (shown in figure 2b).

Lithography is the
principal mechanism for pattern definition in micromachining.
Photosensitive compounds are primarily organic, and do not encompass
the spectrum of materials properties of interest to micro-machinists.
However, as the technique is capable of producing fine features in an
economic fashion, a photosensitive layer is often used as a temporary
mask when etching an underlying layer, so that the pattern may be
transferred to the underlying layer (shown in figure 3a). Photoresist
may also be used as a template for patterning material deposited after
lithography (shown in figure 3b). The resist is subsequently etched
away, and the material deposited on the resist is "lifted off".

The deposition template (lift-off) approach for transferring a
pattern from resist to another layer is less common than using the
resist pattern as an etch mask. The reason for this is that resist is
incompatible with most MEMS deposition processes, usually because it
cannot withstand high temperatures and may act as a source of
contamination.

Once the pattern has been transferred to another layer, the resist
is usually stripped. This is often necessary as the resist may be
incompatible with further micromachining steps. It also makes the
topography more dramatic, which may hamper further lithography
steps.

Alignment

In order to make
useful devices the patterns for different lithography steps that
belong to a single structure must be aligned to one another. The
first pattern transferred to a wafer usually includes a set of
alignment marks, which are high precision features that are used as
the reference when positioning subsequent patterns, to the first
pattern (as shown in figure 4). Often alignment marks are included in
other patterns, as the original alignment marks may be obliterated as
processing progresses. It is important for each alignment mark on the
wafer to be labeled so it may be identified, and for each pattern to
specify the alignment mark (and the location thereof) to which it
should be aligned. By providing the location of the alignment mark it
is easy for the operator to locate the correct feature in a short
time. Each pattern layer should have an alignment feature so that it
may be registered to the rest of the layers.

Figure 4:Use of alignment marks to register subsequent layers

Depending on the lithography equipment used, the feature on the
mask used for registration of the mask may be transferred to the wafer
(as shown in figure 5). In this case, it may be important to locate
the alignment marks such that they don't effect subsequent wafer
processing or device performance. For example, the alignment mark
shown in figure 6 will cease to exist after a through the wafer DRIE
etch. Pattern transfer of the mask alignment features to the wafer
may obliterate the alignment features on the wafer. In this case the
alignment marks should be designed to minimize this effect, or
alternately there should be multiple copies of the alignment marks on
the wafer, so there will be alignment marks remaining for other masks
to be registered to.

Figure 6:Poor alignment mark design for a DRIE through the wafer etch
(cross hair is released and lost).

Alignment marks may not necessarily be arbitrarily located on the
wafer, as the equipment used to perform alignment may have limited
travel and therefore only be able to align to features located within
a certain region on the wafer (as shown in figure 7). The region
location geometry and size may also vary with the type of alignment,
so the lithographic equipment and type of alignment to be used should
be considered before locating alignment marks. Typically two
alignment marks are used to align the mask and wafer, one alignment
mark is sufficient to align the mask and wafer in x and y, but it
requires two marks (preferably spaced far apart) to correct for fine
offset in rotation.

Figure 7:Restriction of location of alignment marks based on equipment
used.

As there is no pattern on the wafer for the first pattern to align
to, the first pattern is typically aligned to the primary wafer flat
(as shown in figure 8). Depending on the lithography equipment used,
this may be done automatically, or by manual alignment to an explicit
wafer registration feature on the mask.

Figure 8:Mask alignment to the wafer flat.

Exposure

The exposure
parameters required in order to achieve accurate pattern transfer from
the mask to the photosensitive layer depend primarily on the
wavelength of the radiation source and the dose required to achieve
the desired properties change of the photoresist. Different
photoresists exhibit different sensitivities to different wavelengths.
The dose required per unit volume of photoresist for good pattern
transfer is somewhat constant; however, the physics of the exposure
process may affect the dose actually received. For example a highly
reflective layer under the photoresist may result in the material
experiencing a higher dose than if the underlying layer is absorptive,
as the photoresist is exposed both by the incident radiation as well
as the reflected radiation. The dose will also vary with resist
thickness.

There are also higher order effects, such as interference patterns
in thick resist films on reflective substrates, which may affect the
pattern transfer quality and sidewall properties.

At the edges of pattern light is scattered and diffracted, so if an
image is overexposed, the dose received by photoresist at the edge
that shouldn't be exposed may become significant. If we are using
positive photoresist, this will result in the photoresist image being
eroded along the edges, resulting in a decrease in feature size and a
loss of sharpness or corners (as shown in figure 9). If we are using
a negative resist, the photoresist image is dilated, causing the
features to be larger than desired, again accompanied by a loss of
sharpness of corners. If an image is severely underexposed, the
pattern may not be transferred at all, and in less sever cases the
results will be similar to those for overexposure with the results
reversed for the different polarities of resist.

Figure 9:Over and under-exposure of positive resist.

If the surface being exposed is not flat, the high-resolution image
of the mask on the wafer may be distorted by the loss of focus of the
image across the varying topography. This is one of the limiting
factors of MEMS lithography when high aspect ratio features are
present. High aspect ratio features also experience problems with
obtaining even resist thickness coating, which further degrades
pattern transfer and complicates the associated processing.

The Lithography Module

Typically
lithography is performed as part of a well-characterized module, which
includes the wafer surface preparation, photoresist deposition,
alignment of the mask and wafer, exposure, develop and appropriate
resist conditioning. The lithography process steps need to be
characterized as a sequence in order to ensure that the remaining
resist at the end of the modules is an optimal image of the mask, and
has the desired sidewall profile.

The standard steps found in a lithography module are (in sequence):
dehydration bake, HMDS prime, resist spin/spray, soft bake, alignment,
exposure, post exposure bake, develop hard bake and descum. Not all
lithography modules will contain all the process steps. A brief
explanation of the process steps is included for completeness.

Dehydration bake - dehydrate the wafer to aid resist
adhesion.

HMDS prime - coating of wafer surface with adhesion promoter.
Not necessary for all surfaces.

Resist spin/spray - coating of the wafer with resist either by
spinning or spraying. Typically desire a uniform coat.

Soft bake - drive off some of the solvent in the resist, may
result in a significant loss of mass of resist (and thickness). Makes
resist more viscous.

Descum - removal of thin layer of resist scum that may occlude
open regions in pattern, helps to open up corners.

We make a few assumptions about photolithography. Firstly, we
assume that a well characterized module exists that: prepares the
wafer surface, deposits the requisite resist thickness, aligns the
mask perfectly, exposes the wafer with the optimal dosage, develops
the resist under the optimal conditions, and bakes the resist for the
appropriate times at the appropriate locations in the sequence.
Unfortunately, even if the module is executed perfectly, the
properties of lithography are very feature and topography dependent.
It is therefore necessary for the designer to be aware of certain
limitations of lithography, as well as the information they should
provide to the technician performing the lithography.

The designer influences the lithographic process through their
selections of materials, topography and geometry. The material(s) upon
which the resist is to be deposited is important, as it affects the
resist adhesion. The reflectivity and roughness of the layer beneath
the photoresist determines the amount of reflected and dispersed light
present during exposure. It is difficult to obtain a nice uniform
resist coat across a surface with high topography, which complicates
exposure and development as the resist has different thickness in
different locations. If the surface of the wafer has many different
height features, the limited depth of focus of most lithographic
exposure tools will become an issue (as shown in figure 10).

Figure 10:Lithography tool depth of focus and surface topology.

The designer should keep all these limitations in mind, and design
accordingly. For example, it is judicious, when possible, to perform
very high aspect patterning step (lithography and subsequent
etch/deposition) last, as the topography generated often hampers any
further lithography steps. It is also necessary for the designer to
make it clear which focal plane is most important to them (keeping in
mind that features further away in Z from the focal plane will
experience the worst focus). The resolution test structures should be
located at this level (as they will be used by the fab to check the
quality of a photo step).